This invention relates to axial flow compressors and, more particularly, to a method for detecting stall onset in axial flow compressor systems using an optical sensor, such as a laser or light probe.
During operation of an aircraft gas turbine, there may occur a phenomenon known as compressor stall, wherein the pressure ratio or compressor operating limit line of the compressor initially exceeds the compressor surge pressure ratio, resulting in a subsequent reduction of compressor pressure ratio and airflow delivered to the engine combustor. Compressor stall may result from a variety of reasons, such as when the engine is accelerated too rapidly, or when the inlet profile of air pressure or temperature becomes unduly distorted during normal operation of the engine, or when over time, erosion has diminished the performance of the compressor airfoils. Compressor damage due to the ingestion of foreign objects or a malfunction of a portion of the engine control system may also result in a compressor stall. If a compressor stall remains undetected and permitted to continue, the combustor temperatures and the vibratory stresses induced in the compressor may become sufficiently high to cause damage to the turbine.
In land-based gas turbines used for power generation, a compressor must be allowed to operate at a high pressure ratio in order to achieve a high machine efficiency. A compressor stall, as identified above with respect to aircraft turbines, may also occur in land-based gas turbines. Similar to the problems faced during the operation of aircraft gas turbines, if a compressor stall remains undetected and permitted to continue in land-based gas turbines, the combustor temperatures and vibratory stresses induced in the compressor may become sufficiently high to cause damage to the turbine.
Several attempts have been made in an effort to determine whether a stall condition is imminent. Typically, the compressor discharge pressure is monitored and when the pressure rapidly drops this provides an indication of that a stall has already occurred. Methods to detect the onset of a compressor surge using pressure sensors have remained elusive. Also, existing experimental techniques require complex mathematical manipulation of very high response pressure signals to anticipate proximity to the surge line. Furthermore, the sensed pressure signals often fail to provide a clear indication of stall onset. These factors make the existing techniques difficult to reliably implement in new or fielded axial flow compression systems. Therefore, it would be desirable to have a reliable stall detection method and apparatus to detect the onset of a compressor surge prior to the event occurrence using an optical sensor, and using the information to initiate the desired control system corrective action.
Accordingly, the present invention is directed to an innovative system and method for detecting the onset of stalls, in axial flow compressor systems, using an optical sensor, for example, a laser or light probe. In this method, at least one optical sensor is provided in the compressor to detect the deflection of airfoil caused by a rotating stall cell. The optical sensor monitors the deflection and vibratory response of a rotating airfoil by estimating the airfoil deflection by the time of arrival of each airfoil, with respect to a reference in a given compressor stage. Measured airfoil deflection values are compared with predetermined steady baseline values in a control algorithm. The electronic control initiates corrective actions, by means of a control algorithm if the measured deflection values are greater than the predetermined values. The predetermined value is estimated analytically and verified experimentally for incorporation into the engine electronic control.
The corrective actions may vary the operating line control parameters which include adjustments to compressor vanes, inlet air heat, compressor air bleed, combustor fuel flow, etc. to operate the compressor at a near threshold level, the level occurring near stall and presumably on a high operating line where maximum efficiency occurs. Preferably, the corrective actions are initiated prior to the occurrence of a compressor surge event and within a margin identified between an operating limit line and the occurrence of a compressor surge event. Corrective actions are iterated until the measured airfoil deflection values lie within acceptable parameters. Stall onset may also precipitate in an axial flow compressor system operating under high aerodynamic loading or high airflow incidence angle.
In one aspect, the present invention provides a method for detecting a stall onset in an axial flow compressor system comprising, (a) providing at least one optical sensor to measure the deflection of an airfoil, the deflection caused by a rotating stall cell; (b) comparing the deflection as measured in step (a) with a predetermined value indicative of a stall; and (c) identifying onset of a stall if the measured deflection is greater than the predetermined value. The method further includes the steps of (d) upon identifying a stall onset as in step (c), feeding the measured deflection to a control system to initiate corrective actions to prevent a compressor surge event; and (e) iterating step (d) until the measured deflection is less than the predetermined value to allow the compressor to operate at higher efficiency. The deflection of the airfoil is measured by measuring the time of arrival of the airfoil with respect to a reference. Further, corrective actions are initiated by varying operating line parameters. Preferably, the operating line parameters are set to a near threshold value.
In another aspect, a stall precursor detector system for an axial flow compressor, comprises at least one optical sensor to detect the deflection of a rotating airfoil of the compressor; means for comparing the deflection with a predetermined value , the predetermined value being indicative of onset of a stall in the compressor; and means for identifying onset of a stall and producing a stall onset signal if said deflection is greater than the predetermined value. The system further comprises a control system for initiating corrective actions to prevent a subsequent compressor surge if the deflection is greater than the predetermined value. The optical sensor measures the time of arrival of an airfoil to determine airfoil deflection. The system further comprises a system selector means for applying the stall onset signal to provide a warning of the stall onset.
In yet another aspect, a stall warning system for an axial flow compressor, comprising at least one optical sensor to detect and measure the deflection of an airfoil of said compressor; means for comparing the deflection with a predetermined value indicative of a stall onset; means for identifying onset of a stall if the measured deflection is greater than the predetermined value; and a feedback control system for initiating corrective actions to prevent a subsequent compressor surge upon identifying a stall onset.
The benefits of the present invention will become apparent to those skilled in the art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention.